1. Field of the Invention
The present invention relates to a human body sound transmission apparatus and method for minimizing signal loss and, more particularly, to a technique that transmits a modulated audio signal through a human body and a carrier wave signal for demodulating the audio signal through the air so as to minimize loss within the audio signal due to a nonlinearity phenomenon occurring in parts of the human body other than the ear, thus increasing the amplitude of the audio signal generated near the ear and improving sound quality.
2. Description of the Related Art
In the related art sound transmission system such as an MP3 player, a portable radio set, or the like, a transmission device transmits an audio signal, desired to be transmitted, through a fixed line, or modulates the audio signal and transmits it wirelessly, and a reception device such as an earphone or the like located near a user's ear receives the signal transferred from the transmission device or receives and then demodulates it, thus allowing the user to hear the sound. However, the related art sound transmission system has a problem in that it must necessarily include the reception device and the user can hear sound only when the reception device is in contact with the user's body, thus causing user inconvenience.
In an effort to improve sound transmission system user convenience, a human body sound transmission technique has been proposed to transmit an audio signal through a human body.
According to the proposed human body sound transmission technique, an audio signal desired to be transmitted is amplitude-modulated so as to be converted into a signal of a ultrasonic band of around 20 kHz or higher, and the modulated audio signal is combined with a carrier wave signal which has been used for the modulation, or is converted into a separate ultrasonic wave signal and then applied to the human body.
The human body (i.e., the user) performs a nonlinear operation, as well as a linear operation proportional to the amplitude of a signal input, with respect to the applied ultrasonic wave signal. The nonlinear operation, with respect to two or more signals, may be modeled as the product of the two signals. Namely, when the audio signal desired to be transmitted is m(t) and the amplitude and frequency of the carrier wave signal are Ac and fc, respectively, the audio signal which has been amplitude-modulated by the carrier wave signal may be represented by Equation 1 and Equation 2 as follows:Acm(t)cos(2πfct)  [Equation 1]Ac cos(2πfct)  [Equation 2]
When the two signals represented by Equation 1 and Equation 2 are converted into ultrasonic wave signals and then applied to the user, the two ultrasonic wave signals are frequency-mixed according to the nonlinear operation of the user while being transmitted through the user's body, to generate a signal represented by Equation 3 shown below, which may be then developed by using a sine formula to be represented by Equation 4 shown below:
                              A          c                ⁢                  m          ⁡                      (            t            )                          ⁢                  cos          ⁡                      (                          2              ⁢              π              ⁢                                                          ⁢                              f                c                            ⁢              t                        )                          ×                  A          c                ⁢                  cos          ⁡                      (                          2              ⁢              π              ⁢                                                          ⁢                              f                c                            ⁢              t                        )                                              [                  Equation          ⁢                                          ⁢          3                ]                                                      1            2                    ⁢                      A            c            2                    ⁢                      m            ⁡                          (              t              )                                ⁢                      cos            ⁡                          (                              4                ⁢                π                ⁢                                                                  ⁢                                  f                  c                                ⁢                t                            )                                      +                              1            2                    ⁢                      A            c            2                    ⁢                      m            ⁡                          (              t              )                                                          [                  Equation          ⁢                                          ⁢          4                ]            
In Equation 4, a first term is a signal of an ultrasonic band, the user cannot hear, and a second term is a signal of an acoustic band, the user can hear. Thus, the foregoing human body sound transmission technique enables the user to hear a transmitted audio signal without having to use a reception device.
However, the human body sound transmission technique has the following problem. That is, as the modulated audio signal and the carrier wave signal are transmitted through the human body, audio signals are generated from all of a user's body parts, as well as from the vicinity of the user's ear, through which the signals pass, due to the nonlinear operation. The audio signals generated from other parts of the human body, excluding the user's ear, become extinct, causing a big loss within the audio signals. Thus, the amplitude of the audio signal generated from the vicinity of the user's ear is very small and its sound quality is not very good.